msm: ipa: Use read memory barrier before reading events

Use read memory barrier before reading event from event ring.
Since ring is shared resource between IPA h/w and IPA driver
dma_rmb is used to ensure memory write is complete before
reading from driver.

Change-Id: I295a701b16362f26d47d4efe5a54364a7ee4d5a6
Signed-off-by: Akshay Pandit <pandit@codeaurora.org>